Process for purifying benzene and toluene by extractive azeotropic distillation

ABSTRACT

Process for separating aromatics from hydrocarbon mixtures (containing the same) by combined extractive and azeotropic fractional distillation thereof in the presence of a liquid alkyl aliphatic amide as extraction (solvent) agent, comprising introducing said mixture at a first level of a distillation column, introducing the extraction (solvent) agent at a second level of the distillation column above the first one, introducing steam as an azeotropic agent in the distillation column above said second level, withdrawing the aromatics and the extraction (solvent) agent from the bottom of the column and the nonaromatic hydrocarbons and the steam from the top thereof.

United States Patent Mikitenko et al.

PROCESS FOR PURIFYING BENZENE AND TOLUENE BY EXTRACTIVE AZEOTROPIC DISTILLATION Inventors: Paul Mikitenko, Chatou; Georges Cohen, Rueil-Malmaison; Lionel Asselineau, Paris, all of France Assignee: Institut Francais du Petrole, des

Carburants et Lubrifiants, Rueil-Malmaison, France Filed: Mar. 20, 1973 Appl. N0.: 343,108

Foreign Application Priority Data Mar. 20, 1972 France 72.09763 US. Cl. 203/53; 203/60; 203/96; 260/674 A; 260/674 SE Int. Cl B0ld 3/34; C070 15/28 Field of Search 203/53, 95-97, 203/60; 260/674 R, 674 A, 674 SE; 208/313,

References Cited UNITED STATES PATENTS 8/1957 Sureano 203/96 2,859,155 11/1958 Cichelli et a1 203/96 2,957,811 10/1960 Geiser 203/53 3,114,783 12/1963 Butler et al. 203/60 3,151,046 9/1964 Larson 203/96 3,328,267 6/1967 Muller 203/60 3,684,665 8/1972 Abe et al. 203/60 FOREIGN PATENTS OR APPLICATIONS 2,012,330 10/1970 Germany 260/674 SE Primary Examiner-Jack Sofer Attorney, Agent, or Firm-Millen, Raptes & White [57] ABSTRACT Process for separating aromatics from hydrocarbon mixtures [containing the same] by combined extractive and azeotropic fractional distillation thereof in the presence of a liquid alkyl aliphatic amide as extraction [solvent] agent, comprising introducing said mixture at a first level of a distillation column, introducing the extraction [solvent] agent at a second level of the distillation column above the first one, introducing steam as an azeotropic agent in the distillation column above said second level, withdrawing the aromatics and the extraction [solvent] agent from the bottom of the column and the non-aromatic hydrocarbons and the steam from the top thereof.

10 Claims, 1 Drawing Figure NON-AROMATICS VAPOR IZER EXTRACTIVE AROMATICS 8 1 1 .11 4 CONDENSER AROMATICS SIMPLE RECTIFICATION COLUMN AROMATICS PLUS ORGANIC SOLVENT Pmminmzm s V 3,884,769

3 AROMATICS 8 NON-AROMATICS r 11 VAPORIZER EXTRACTIVE CONDENSER AZEOTROPIC DISTILLATION COLUMN (N AROMATICS ORGANIC SOLVENT V CH I'IC FEED I SIMPLE RECTIFICATION COLUMN AROMATICS PLUS ORGANIC SOLVENT PROCESS FOR PURIFYING BENZENE AND TOLUENE BY EXTRACTIVE AZEOTROPIC DISTILLATION This invention concerns a process for separating aromatic hydrocarbons, particularly benzene and/or toluene, from mixtures thereof, with paraffinic branched or unbranched hydrocarbons, having close boiling points, by combine extractive and azeotropic distillation.

The extractive agent has a higher dissolving power for the hydrocarbons and a good selectivity for the aromatics, whereas the azeotropic agent has a limited dissolving power for the hydrocarbons and the property of forming an azeotropic system with at least one of the non-aromatic hydrocarbons which have to be separated. These two agents also have such boiling points as to be discharged each from a different end portion of the distillation column: one from the bottom of the column with the aromatic hydrocarbons and the other at the top of the column with the non-aromatic hydrocarbons.

By this process, there can be obtained pure benzene and/or toluene from, for example, benzene cuts and/or toluene cuts obtained from reaction products containing ofaromatic hydrocarbons starting from saturated or unsaturated gasolines (which have been subjected to a previous partial hydrogenation, so as to limit the impurities substantially to the saturated hydrocarbons) and subjected to pyrolysis, cracking, steam cracking or catalytic reforming reactions.

By benzene cut, there is meant a mixture of benzene and hydrocarbons having a lower boiling point of at least about 65C and an upper boiling point of at most 102C. For example, this cut may consist of a mixture of benzene with saturated hydrocarbons having essentially 6 to 8 carbon atoms. However, the invention may also be applied to benzene cuts containing lighter hydrocarbons.

By toluene cut, there is meant a mixture of toluene with saturated hydrocarbons having lower and upper boiling points of about 102C to about 120C.

BACKGROUND OF THE INVENTION It is well known that pure benzene and/or toluene cannot be obtained from these cuts by mere distillation, since these aromatic hydrocarbons either form'azeotropes with other non-aromatic hydrocarbons which are present, or have a boiling point which is so close thereto that efficient separation is not feasible.

The liquid-liquid extraction process is used when it is desired to selectively extract a relatively large number of hydrocarbons of the same chemical family, for example, benzene, toluene, xylene, ethylbenzene etc the extractive distillation is preferred when it is desired to selectively extract a more limited number of compounds of the same chemical family. for example, benzene and/or toluene. I

A large number of various extraction solvents, or solvent mixtures, have been proposed for carrying out these two techniques. They are generally the first member of mono or bifunctional polar chemical families.

In liquid-liquid extraction processes, it has been contemplated, for example, to use such solvents as alkylated, aliphaticor internal amides (lactams), for example methylformamide and N-methylpyrrolidone, aldehydes, for example furfural, polyols for example glycol, sulfoxides for example dimethylsulfoxides, sulfones, for example sulfolane, lactones, for example 'y-butyrolactone, oxazines, for example morpholine, aliphatic nitriles etc For extractive distillation in, industrial plants for aromatic purification, there are employed as the extractive agents phenol, aniline, sulfolane, formylmorpholine, N-methylpyrrolidone, dimethylformamide, or furfural etc Although each extractive agent is generally selected so that its boiling temperature is significantly higher than that of the less volatile saturated hydrocarbon of the hydrocarbon mixture to be separated, it is impossible to avoid the formation of a hydrocarbon-solvent azeotrope which'results in a significant loss of the extractive agent at the top of the extractive distillation column. Accordingly, it is necessary to recover the solvent carried along, thus requiring additional operations: various processes have been contemplated in order to recover the solvent, for example the process of washing with water counter-currently with the distillate in a supplemental tower, or, still by way of example, a process consisting of injecting liquid water in the duct through which the distillate is withdrawn from the extractive distillation column (the water being even, optionally, in this type of process, introduced at the top of the extractive distillation column); such processes do not provide .for the complete elimination of the extraction agent from the distillate, and the solvent, progressively, accumulates in the distillate, so that it becomes necessary to recover it by means of costly and lengthy distillations. Similarly, distillations are also required for separating the recovered agent from the water used for washing the distillate and these distillations are not only lengthy but they are also detrimental to the stability of certain agents which are sensitive to attack by liquid water.

SUMMARY pable of forming anazeotrope with said saturated hy- "drocarbon. For the process to be carried out advantageously and without loss of solvent at the top of the column, in such a purification as that the benzene and/or toluene the extractive agent is employed, in conjunction with an agent which preferably consists of steam (this agent being called associated solvent or azeotropic agent although it is used in the vapor form and not in the liquid form) according to a distillation procedure which will be called hereinafter extractive .azeo- I tropic distillation.

Among the numerous selective solvents, only the compounds pertaining to the family of the alkyl aliphatic amides and more particularly the first members of this family containing from 2 to carbon atoms per molecule (in particular dimethylformamide and dimethylacetamide), are particularly well adapted for purifying benzene and/or toluene according to the process of the invention making use of steam as associated solvent.

These solvents are well known as efficient separating agents of hydrocarbons having different degrees of saturation. However, they also have the property of forming azeotropes with saturated hydrocarbons of 6,7 or 8 carbon atoms, and accordingly their use in a conventional extractive distillation for purifying benzene and- /or toluene would result in an unavoidable loss of solvent which would be carried along at the top of the column with the impurities having 6,7 or 8 carbon atoms of the benzene and/or toluene.

When proceeding according to the invention, with the associated solvent, this loss of the extraction agent is avoided. More generally, the associated solvent must have a boiling temperature lower than that of the extraction solvent, must not form an azeotrope with the latter and must, preferably, be miscible to a low extent with non-aromatic hydrocarbons of the benzene and/or toluene fractions. For example, the non-aromatic hydrocarbons must dissolve less than 1 percent and preferably less than 0.1 percent by weight of said associated solvent.

According to the invention, pure benzene and/or toluene are obtained from mixtures containing the same together with cycloparaffinic (naphthenic) or paraffinic branched or unbranched hydrocarbons, by subjecting said mixture to distillation in the presence of a solvent system consisting, on the one hand, of an extraction solvent and, on the other hand, of the associated solvent, such as defined supra.

The manipulative steps of process are such that the mixture of hydrocarbons to be separated is introduced in the distillation zone, at an intermediate point thereof and the extraction solvent is introduced at a point of the distillation zone, preferably above the point of introduction of the hydrocarbon mixture, and in the associated solvent is introduced, in the vapor form, at a point in the distillation zone above the point of introduction of the extraction solvent.

According to a further embodiment of the process, the top product of the distillation zone or distillate is condensed and the resulting condensate is separated in two liquid phases, a first phase of which contains the non-aromatic hydrocarbons and the second phase, the associated solvent, the first and second phase being withdrawn separately, the bottom product containing aromatic hydrocarbons and the extraction solvent is discharged from said distillation zone and the solvent is separated from the aromatic hydrocarbons in a known manner, so as to obtain, on the one hand, the recovered extraction solvent, and on the other hand the aromatic hydrocarbons.

According to a preferred embodiment, the efficiency of the fractionation is increased by feeding back to the distillation column, as reflux, a portion of the first condensed phase, the other portion being merely withdrawn. The point of introduction of this reflux is preferably above the point of introduction of the azeotropic agent.

It is also advantageous to feed back to the column at least one portion of the condensed second phase, after vaporization, so as to feed said column with vaporized associated solvent. Finally, the recovered extraction solvent, after its separation from the aromatics, is advantageously recycled to the column.

The aromatic hydrocarbons may be separated from the extraction solvent by distillation or liquid-liquid reextraction of the aromatics by means of a third solvent, such as a liquid paraffinic hydrocarbon, which can be later separated by distillation from said aromatic hydrocarbons, this being the case, for example, of liquid butane, pentane or heavy cuts such as gas-oil or kerosene.

It is essential, according to the invention, to make use of the associated solvent, water, in the vapor phase. As a matter of fact, if liquid water were injected on the path followed by the vapors issued from the extractive distillation column, at the condenser inlet, the carrying along of the extraction solvent vapors could not be avoided: after passage through the condenser, there would be obtained two phases, an aqueous phase and a hydrocarbon phase, which would contain substantial amounts of said solvent.

On the contrary, by making use of the vaporized associated solvent according to the invention, the extraction solvent vapors are substantially not carried away from the extractive distillation column.

BRIEF DESCRIPTION OF DRAWING The accompanying drawing illustrates a preferred embodiment for carrying out the invention. In the following, the associated agent is water and the extraction solvent is a convenient organic solvent.

The hydrocarbon mixture containing the benzene and/or toluene to be extracted is sent, preferably at a temperature close to its bubble point, through pipe 1 to column CI. The organic solvent is introduced into said column through pipe 6, opening thereinto at a level above that of the initial hydrocarbon mixture, at a temperature close to that prevailing in the column at the same level under predetermined operating conditions. The ratio by volume of the solvent to the hydrocarbons feed is advantageously selected from 0.4 to 15, and preferably from 1 to 6. The organic solvent, which is the less volatile compound, mainly flows in the liquid form towards the bottom of column CI, carrying along therewith the aromatics to which it imparts a modified volatility with respect to the paraffinic or naphthenic impurities which originally are present therewith.

The mixture solvent-aromatics is discharged from column CI through pipe 7 and conveyed to column CII wherefrom, by conventional distillation, the aromatics are separated at the top through line 8, the condenser 11 and line 13, and at the bottom, the regenerated solvent which is recycled to column Cl through line 6. A portion of the aromatic hydrocarbons may be fed back to the colum CII through pipe 12. According to an alternative embodiment of the process, and in the case of simultaneous purification of benzene and toluene, the effluent from the bottom of column CI, comprising the organic solvent and the aromatics, is sent to column CII wherefrom purified benzene is discharged at the top and the organic solvent containing the toluene, from the bottom. This last mixture is sent to a column Clll, not shown, at the top of which the purified toluene is obtained and from the bottom of which the organic solvent is withdrawn and recycled to column Cl through pipe 6.

A few plates above the points of introduction of the organic solvent, vaporized water, slightly superheated, is injected (through line 5 and vaporizer 9). It is convenient to make use of 5 to 50 percent, preferably, 10 to 35 percent by weight of water with respect to the nonaromatic hydrocarbons of the treated charge. The amount of steam will be preferably lower than that which would lead to a water condensation in order to avoid that liquid water be admixed with the extraction solvent.

The water forms with the hydrocarbons present, i.e. mainly the non-aromatic ones, an azeotropic mixture which is more volatile with respect to the organic solvent than the hydrocarbons alone, which azeotropic mixture distills at the top of the column and is discharged through pipe 2. This effluent is condensed in unit 10 and decanted in two liquid phases in the decanter D. A portion of the upper phase, consisting of hydrocarbons, is fed back through pipe 3 to column Cl, as reflux, while the other portion is discharged from the unit through pipe 4. The lower phase, consisting essentially of water, may be withdrawn from the unit, or recycled to column CI through pipe 5, under the hereabove defined conditions.

The associated solvent is generally introduced, in the distillation column, for a particularly efficient operation of the process, at a level above the point of introduction of the selective solvent but below that of the distillate reflux.

When operating according to the invention, it appears that almost the entirety of the organic solvent is collected at the column bottom with the aromatics and that the non-aromatic hydrocarbons, discharged at the top of the column, are substantially free of organic solvent, this being not the case when using a conventional extractive distillation, by means of the same organic solvent alone. The following table gives approximative data on the solvent loss at the top of the column, obtained when using dimethylforrnamide in the treatment of various aromatic fractions.

It has also been observed that the aromatic solvent is collected at the bottom of the column, together with the purified aromatic hydrocarbons, substantially in the absence of the injected water which is mainly discharged from the top of the column.

Treated Dimethylformamide loss at the top aromatic fraction of the column by weight with respect to the distillate) Conventional extrac- Process accortive distillation ding to the invention Benzene fraction 4.5 0.0l Toluene fraction 7.4 0.0l Benzene and toluene fraction 5.1 0.0l

A second advantage is due to the fact that the extraction solvent is not shared between the top and the bottom of the separation column: all the solvent is used for purifying the aromatics and, accordingly, for the same degree of purity, the amount of solvent to be introduced in the column is lower.

Another advantage is due to the fact that the associated solvent, i.e. water, facilitates the removal of the impurities from the aromatics.

Still another advantage is in the fact that the associated solvent and the extraction solvent are never in direct contact with each other in the liquid phase, in the hot portions of the apparatus. Accordingly, any possible chemical reactions between the two solvents is avoided. It is well known, for example, that the dialkyl amides may be slightly hydrolyzed in the presence of water, in the liquid phase, at a temperature higher than 120C. According to the process of the invention, the water is in the vapor phase and the amide in the liquid phase in a zone where the temperature is about C. The only liquid phase contact between the amide and the water which might casually occur in the case of a improper operation of the apparatus, would be in the decanter where the temperature is always lower than 70C and may be selected as low as desired.

The following examples illustrate the invention but are not intended to limit the same in any way.

EXAMPLE 1 In a distillation column, 4 meters high, consisting of 3 meters of elements of the Oldershaw type having a 2.5 cm diameter and of an adiabatic element with a Dixon filling system, of the same diameter, there are injected, at the respective levels of 1m, 2.5m, and 3m from the column bottom:

1. A benzene cut whose composition is given in table 1, at a flow rate of 240 cc/h, brought to a temperature of 82C,

2. dimethylforrnamide, at a flow rate of 370 cc/h,

brought to a temperature of C,

3. slightly superheated vaporized water, at a flow rate of 7.5 cc/h.

The distillate, which is recovered from the top of the column, at a temperature of 71C, is condensed and decanted in two phases: a lower phase consisting of water which is recycled to the column at the previously defined level and an upper phase whose composition is indicated in table 1, consisting essentially of nonaromatic impurities of the benzene cut, a portion of which, ie 54 g/h, is withdrawn and discharged from the system and the other portion of which is fed back to the top of the column, also at a rate of 54 g/h, as reflux.

From the column bottom where the prevailing temperature is 1 15C, there is withdrawn a mixture consisting essentially of dimethylforrnamide and benzene which is fed at a rate of 550 cc/h to the middle of a second column 2 meters high, consisting of Oldershaw elements of a 2.5 cm diameter. From the top of said second column, purified benzene is withdrawn (table 1, column 3), one half of which is fed as reflux while, from the bottom of the same column, dimethylforrnamide is withdrawn and thereafter recycled, at a rate of 350 g/h,

to the first column.

TABLE 1 COMPOSITION BY WEIGHT OF THE BENZENE FRACTION AND OF THE TWO EFFLUENTS OF THE DISTILLATION SYSTEM This example is only given by way of comparison and forms no part of the invention.

Example 1 is repeated except that the amount of water introduced in example 1 as vapor, is now introduced in the liquid form, in the extractive distillation column, at the level where, in example 1, water was injected in the form of steam.

These conditions have resulted not only in an accumulation of solvent in the upper phase of the distillate, but also in a progressive accumulation of water in the solvent which is the opposite of the desired result.

In an attempt to avoid these inconveniences, there have been placed in the extractive distillation column, between the level of the liquid injection and the level of the solvent injection, a decanter plate, specially designed for removing water from the system. The aqueous phase withdrawn from this decanter plate, had in these conditions a to 20 percent content of extraction solvent and the upper phase of the distillate contained more than 1 percent of said solvent, which is contrary to the desired result.

EXAMPLE 1B This example is also given by way of comparison; it forms no part of the invention.

Example 1 is repeated, while introducing the indicated water amounts in the liquid form, not in the column itself, but in the pipe between the top of the column and the condenser of the vapors issued from said column. In these conditions, the upper phase of the distillate contained more than 1 percent of the extraction solvent, which is contrary to the desired result and makes necessary, after a long working period, a distillation for recovering the solvent carried away from the column.

EXAMPLE 1C EXAMPLE 2 In the same column as that described in the example 1, there are introduced, at the respective levels of 1m, 2.5m and 3m from the column bottom:

l. A toluene cut, whose composition is given in table II (first column), at a flow rate of 180 cc/h and preheated to a temperature of 180C,

2. dimethylacetamide at a rate of 380 cc/h and preheated at a temperature of 105C,

3. vaporized water slightly superheated, at a rate of While operating in conformity with the preceding example, a portion of the hydrocarbon phase (23 g/h) is withdrawn from the distillate (table 11, column 2), while the other portion (also 23 g/h) is recycled as reflux and the aqueous phase is recycled to the above defined level.

From the column bottom, there is also withdrawn a mixture of dimethylacetamide with toluene which is sent to a second distillation column from the top of which, the purified toluene is discharged (analysis by weight given in table II, column 3) while the dimethylacetamide discharged from the bottom is fed back to the first column.

In a column similar to that described in example 1, there are introduced, respectively at 1m, 2.5m and 3m from the column bottom:

1. A benzene and toluene fraction whose composition is given in table III (first column), at a rate of 123 cc/h and preheated at a temperature of C,

2. dimethylformamide, at a rate of 635 cc/h and, at a temperature of C,

3. vaporized water slightly superheated at a rate of 9.5 cc/h.

The water and the non-aromatic hydrocarbons are recovered at the top of the column where the prevailing temperature is 75C, and they are condensed and separated by decantation in two phases. The lower phase, consisting of water, is recycled to the column after vaporization while a portion (33.2g) of the upper phase is withdrawn from the system and the other portion (33 g) is recycled to the top of the column as reflux. The composition of the withdrawn hydrocarbon phase is given in table Ill, column 2.

From the column bottom, there is withdrawn dimethylformamide containing the extracted benzene and toluene. This mixture is sent to a second distillation column in which benzene and toluene are separated at the top while the regenerated solvent is withdrawn from the bottom and recycled to the first column in the above-defined conditions. The composition of the purified aromatics is given in table Ill, column 3.

A mere rectification is sufficient thereafter for separating benzene from toluene.

What we claim is:

1. A process for separating at least one aromatic hydrocarbon selected from the group consisting of benzene and toluene, from mixtures containing the same together with at least one saturated paraffinic or cycloparaffinic hydrocarbon, in'which said mixtures are subjected to an extractive azeotropic distillation in the presence of an extraction solvent in the liquid state, said extraction solvent being an alkyl aliphatic amide, said process comprising introducing the mixture of hydrocarbons to be separated to an intermediate point of a distillation zone, introducing said extraction solvent to a point of the distillation zone above said intermediate point, introducing steam at a point of the distillation zone above the point of introduction of the extraction solvent, withdrawing from the bottom of said distillation zone the aromatic hydrocarbons and the extraction solvent, and withdrawing from the top of said distillation zone, a distillate substantially free of the ex- 3. A process according to claim 1, in which steam is used in an amount of at least 10 percent by weight and at most 35 percent by weight of the non-aromatic hydrocarbons contained in the feed subjected to the treatment.

4. A process according to claim 1, in which the hydrocarbons in admixture with said at least one aromatic hydrocarbon are saturated hydrocarbons having from 5 to 8 carbon atoms per molecule.

5. A process according to claim 1, in which the ratio by volume of the amount of extraction solvent to the hydrocarbon feed is from 0.4 to 15.

6. A process according to claim 1, in which said mixtures subjected to extractive azeotropic distillation are gasolines produced by pyrolysis, cracking, steam cracking or catalytic reforming.

7. A process according to claim 1, in which the distillate withdrawn'from the top of the distillation zone, is condensed, the obtained condensate is separated in two liquid phases, a first phase containing the non-aromatic hydrocarbons and asecond phase containing the condensed water vapor, in which the first and the second phase are separately withdrawn, at least a portion of the first phase is fed back to the distillation zone, as reflux, and at least a portion of the condensed vapor water is fed back, after revaporization to steam, to the drocarbons contained in the feed to be treated.

distillation zone.

8. A process according to claim 7, in which the steam is introduced into the distillation column at a level above the point of introduction of the extraction solvent and below that of said reflux.

9. A process according to claim 1, in which the ratio by volume of the amount of extraction solvent used to the hydrocarbon feed is from 1 to 6.

10. A process according to claim 9, comprising a further step of separating the aromatic hydrocarbons from the extraction solvent by distillation.

UNITED STATES PATENT OFFICE QERHFICATE OF (IGRREQTKON PATENT NO. 3,884,769

DATED I May 20, 1975 rNvENTOktS) 1 Paul MIKITENKO et 9.1

It is'certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Title Page, in the ABSTRACT, delete all brackets and 5 words therein.

0 Claim 7, column 10, line 24, delete "phase" and insert -phases--;

column 10, line 26, after "condensed" insert 9 -water-;

gigned and Ecalcd this sixteenth Day of September1975 {SEAL} Arrest:

RUTH c. MASON c. MARSHALL DANN Aflesfl'ng ()fj'i r (ommissr'rmer ufParenls and Tradema 6* 

1. A PROCESS FOR SEPARATING AT LEAST ONE AROMATIC HYDROCARBON SELECTED FROM THE GROUP CONSISTING OF BENZENE AND TOLUENE, FROM MIXTURES CONTAINING THE SAME TOGETHER WITH AT LEAST ONE SATURATED PARAFFINIC OR CYCLOPARAFFINIC HYDROCARBON, IN WHICH SAID MIXTURES ARE SUBJECTED TO AN EXTRACTIVE AZEOTROPIC DISTILLATION IN THE PRESENCE OF AN EXTRACTION SOLVENT IN THE LIQUID STATE, SAID EXTRACTION SOLVENT BEING AN ALKYL ALIPHATIC AMIDE, SAID PROCESS COMPRISING INTRODUCING THE MIXTURE OF HYDROCARBONS TO BE SEPARATED TO AN INTERMEDIATE POINT OF A DISTILLATION ZONE, INTRODUCING SAID EXTRACTION SOLVENT TO A POINT OF THE DISTILLATION ZONE ABOVE SAID INTERMEDIATE POINT, INTRODUCING STEAM AT A POINT OF THE DISTILLATION ZONE ABOVE THE POINT OF INTRODUCTION OF THE EXTRACTION SOLVENT, WITHDRAWING FROM THE BOTTOM OF SAID DISTILLATION ZONE THE AROMATIC HYDROCARBONS AND THE EXTRACTION SOLVENT, AND WITHDRAWING FROM THE TOP OF SAID DISTILLATION ZONE, A DISTILLATE SUBSTANTIALLY FREE OF THE EXTRACTION SOLVENT AND CONTAINING NON-AROMATIC HYDROCARBONS AND STEAM.
 2. A process according to claim 1, in which steam is used in an amount of at least 5 percent by weight and at most 50 percent by weight of the non-aromatic hydrocarbons contained in the feed to be treated.
 3. A process according to claim 1, in which steam is used in an amount of at least 10 percent by weight and at most 35 percent by weight of the non-aromatic hydrocarbons contained in the feed subjected to the treatment.
 4. A process according to claim 1, in which the hydrocarbons in admixture with said at least one aromatic hydrocarbon are saturated hydrocarbons having from 5 to 8 carbon atoms per molecule.
 5. A process according to claim 1, in which the ratio by volume of the amount of extraction solvent to the hydrocarbon feed is from 0.4 to
 15. 6. A process according to claim 1, in which said mixtures subjected to extractive azeotropic distillation are gasolines produced by pyrolysis, cracking, steam cracking or catalytic reforming.
 7. A process according to claim 1, in which the distillate withdrawn from the top of the distillation zone, is condensed, the obtained condensate is separated in two liquid phases, a first phase containing the non-aromatic hydrocarbons and a second phase containing the condensed water vapor, in which the first and the second phase are separately withdrawn, at least a portion of the first phase is fed back to the distillation zone, as reflux, and at least a portion of the condensed vapor water is fed back, after revaporization to steam, to the distillation zone.
 8. A process according to claim 7, in which the steam is introduced into the distillation column at a level above the point of introduction of the extraction solvent and below that of said reflux.
 9. A process according to claim 1, in which the ratio by volume of the amount of extraction solvent used to the hydrocarbon feed is from 1 to
 6. 10. A process according to claim 9, comprising a further step of separating the aromatic hydrocarbons from the extraction solvent by distillation. 